Helical coil springs for treating fractures have already become known from EP 374 088 A1 and DE 38 35 682 A1. In certain cases, the bone substance in which a bore wire is to be inserted is so soft that appropriate fixation is difficult.
It is the object of the invention to provide an osteosynthesis aid for treating upper arm fractures close to the head, which requires little operative expenditures, on the one hand, and ensures sufficient anchorage in the head of humerus, which is soft--in particular with an advanced age--, on the other hand. This object is achieved in that the helix for its lateral insertion into the medullary space is reversibly deformable in the longitudinal direction by up to 90.degree. over its entire length. Said deformability is feasible or possible within an axial or longitudinal distance corresponding to twice the diameter of the helix. Furthermore, the cross section of the helical wire is triangular, rhombic or, in a known manner, round.
As already mentioned above, helically designed coil springs for treating fractures in the shaft region of long tubular bones are known. They stabilize through the long stretched-out contact with the wall of the medullary canal, exhibit a high flexural strength as well as longitudinal rigidity and are introduced in the axial direction in the sense of medullary pins. This is often feasible only by opening adjacent joints, which constitutes a considerable surgical expense and involves the risk of destruction of the articular segment, particularly in the osteoporotic bone. It is clearly less demanding, if the implant can be introduced through an access hole provided in the shaft off joints and fractures. In accordance with the invention, this is achieved in that the helix is imparted special material properties allowing for a uniform reversible variation in the pitch of the thread after introduction, in order to render feasible a change of direction by up to 90.degree.. The helix rigidity resulting from the configuration and material properties, on the other hand, must be so high as to enable a uniform rotary force flux without permanent deformation. As the tip of the helix has reassumed its original form after a directional change, the implant is bored into the spongy bone of the humeral head in the sense of a coreless bone screw designed in the manner of a cork screw.
The deformation remaining at the entry bore hole, which is reversible per se, guarantees the desired protection against implant migration.